1. Field of the Invention
The present invention relates to the fabrication of gallium arsenide (GaAs) microwave monolithic integrated circuits (MMIC), and, more particularly, to forming metal contact layers in via holes in such devices.
2. Description of Related Art
During the fabrication of GaAs MMIC devices, via holes are formed on the backside of the GaAs wafer and extending through to the frontside of the wafer, which is secured to a mounting substrate. A metal, typically a titanium/platinum/gold multilayer structure, is blanket-deposited on the backside of the wafer and through the via holes, to make contact with the MMIC devices, which are formed on the frontside of the GaAs wafer.
After testing, each MMIC device is separated from the others by dicing, and each device is mounted on metal carriers, such as molybdenum, using a gold-tin (AuSn) eutectic bonding process. However, a problem with the bonding process is that the AuSn liquid solder has a tendency to fill up the via holes during the bonding process, which results in cracking of the via holes and tin migration through the via holes upon cooling of the AuSn solder. The cracking and tin migration degrades reliability and reduces yield of these GaAs MMIC devices significantly. Indeed, losses approaching 40% are not uncommon.
Thus, a need remains for a process for forming metal contacts in via holes while preventing filling of the via holes by AuSn solder during the eutectic AuSn bonding process.